This invention relates generally to integrated circuits and, more particularly, to nonvolatile memory arrays that use magnetic memory elements with flux concentrators.
During the 1950s and 1960s, magnetic core memories were the predominant storage technology for the working memory of computing systems. The magnetic core memory technology was costly. In the 1970s, magnetic core memories were replaced with integrated circuits, including static random access memory (SRAM) and including dynamic random access memory (DRAM). Non-volatile memories, such as FLASH memory for example, have been developed to address the problem of data volatility. Because of the rapid advancement in semiconductor density coupled with the advent of the DRAM cell, magnetic storage technology was not used for high-speed on-line memory, but rather was left to be used for low-cost, high-density memory in the form of various disk drive technologies.
The semiconductor industry continuously strives to reduce the size and cost of memory, increase the speed for accessing memory, and improve the reliability of memory. One particular problem confronting the semiconductor industry is that of reducing the size of the memory cell in a Random Access Memory (RAM).
Magnetic storage such as Magnetic Random Access Memory (MRAM) technology has been proposed as a replacement or supplement to the DRAM. In the MRAM structures that are being proposed, the capacitor storage element of the DRAM cell is replaced by a magnetic element that has a magnetic moment and is characterized by a predominant or easy axis of magnetization. In the absence of an external magnetic field, the magnetic moment is oriented along the easy axis of magnetization in one of two stable states. In magnetoresistance technology, one of the stable states for the magnetic moment of the magnetic element is a high resistance state and the other of the stable states is a low resistance state.
A flux concentrator is a material that has a high permeability and that is magnetically soft. Flux concentrators concentrate magnetic flux into a desired area, and have been used in MRAM structures to minimize the current required to set the magnetization of the memory element. One example of a flux concentrator is high permeability cladding material which has been incorporated on the outside faces of conductors to focus the magnetic flux toward the magnetic element and to provide shielding from stray electric fields.
Although flux concentrators minimize the current required to set the magnetization of the memory element, they have a non-zero remanence. One definition of remanence is the magnetic inductance remaining in a magnetized substance that is no longer under external magnetic influence. Thus, flux concentrators are capable of magnetically biasing the magnetic elements in the MRAM. The magnetic biasing effect of the remanence is capable of significantly changing the amount of current required to write to the bit because the magnetic induction attributable to the remanence is summed to the current-induced magnetic flux.
Less current is required to change the stable state of the magnetic moment when the biasing effect of remanence is parallel to the current-induced magnetic flux and antiparallel to the magnetic moment of the cell such that half-select errors are capable of being more problematic. A half-select error occurs when the current in one of the lines (bit or word lines) is sufficient to write data to the memory element. That is, when a selected memory element is written, a half-select error causes other memory element(s) in the same row and/or the same column to be unintentionally written.
More current is required to produce a sufficiently large magnetic field to overcome the biasing effect of remanence when the biasing effect is antiparallel, or opposing, to the current-induced magnetic flux and parallel to the magnetic moment of the cell. If the biasing effect of remanence is too strong, the selected memory storage element is not able to be written.
Therefore, there is a need in the art to overcome the problems attributable to remanence in flux concentrators in magnetic memory elements.
The above mentioned problems are addressed by the present subject matter and will be understood by reading and studying the following specification. The present subject matter provides magnetic memory elements with low remanence flux concentrators. Flux concentrators concentrate magnetic flux into a desired area, and minimize the current required to set the magnetization of the memory element. The flux concentrator is provided with a large anisotropy to provide the flux concentrator with an easy axis of magnetization that is generally aligned with the conductor and a hard axis of magnetization that is generally orthogonal to the easy axis of magnetization. When a current-induced magnetic field is removed, the magnetization of the flux concentrator realigns with the easy axis of magnetization. As such, remanence, along with the associated biasing magnetic field to the memory element, is reduced.
One aspect of the present subject matter is a memory cell. One memory cell embodiment includes a magnetic memory element and a flux concentrator operably positioned with respect to a conductor. The conductor is adapted to provide a current-induced magnetic flux to the magnetic memory element. The flux concentrator includes an easy axis of magnetization aligned with the conductor and a hard axis of magnetization orthogonal to the easy axis of magnetization.
One aspect provides a method of writing to a magnetic storage device. According to one embodiment, a first conductor is energized to provide a first current-induced magnetic field through a first flux concentrator in a hard axis of magnetization direction. The first current-induced magnetic field is provided to the magnetic storage device. A second conductor is energized to provide a second current-induced magnetic field through a second flux concentrator in a hard axis of magnetization direction. The second current-induced magnetic field is provided to the magnetic storage device. The combination of the first current-induced magnetic field and the second current-induced magnetic field is sufficient to move a magnetic moment of the magnetic storage device from a first stable state to a second stable state. When the current-induced magnetic field is removed, the magnetization of the first and second flux concentrators realigns with their easy axis of magnetization.
One aspect provides a method of forming a memory array. According to one embodiment, at least one first flux concentrator is formed with an easy axis of magnetization. At least one first conductor is formed to be operably positioned with respect to the first flux concentrator and aligned with the easy axis of magnetization. At least one magnetic memory element is formed to be operably positioned with respect to the first flux concentrator and the first conductor so as to receive a first current-induced magnetic field. At least one second conductor is formed to be operably positioned with respect to the magnetic memory element so as to provide the magnetic memory element with a second current-induced magnetic field from the second conductor. The second current-induced magnetic field is relatively orthogonal to the first current-induced magnetic field. At least one second flux concentrator is formed to be operably positioned with respect to the second conductor. The second flux concentrator is formed with an easy axis of magnetization aligned with the second conductor.
These and other aspects, embodiments, advantages, and features will become apparent from the following description of the invention and the referenced drawings.